1. Technical Field
This invention relates to bearing compartments within gas turbine engines in general, and to bearing compartment lubricant seals, in particular.
2. Background Information
Modern gas turbine engines often include multiple coaxial spools connecting turbine stages to compressor and fan stages. Depending upon the application, the coaxial spools may co-rotate or counter-rotate, and may differ in rotational velocity. The difference in relative rotational velocity between co-rotating coaxial spools, for example, is almost always less than that of counter-rotating spools because the latter spools are rotating in opposite directions. Co-rotating and counter-rotating applications, consequently, are generally considered to be low rotational velocity and high rotational velocity applications, respectively. Bearings disposed between the two spools maintain concentricity between the spools and facilitate relative motion. Oil, or synthetic lubricant, is fed into the bearing compartments to lubricate and cool the bearings.
Under present design practice, oil is maintained within the bearing compartment by a combination of seals and high pressure air bled off an upper stage compressor stage, sometimes referred to as "buffer air". Compartments adjacent the bearing compartment are maintained at a higher pressure than the bearing compartment (i.e., positively pressurized) and the buffer air is allowed to leak into the bearing compartment. The inflow of buffer air prevents outflow of oil. The buffer air and any oil that maybe entrained within the air is subsequently scavenged off, passed through a oil/air separator, and cycled back into the bearing compartment.
Contact seals and labyrinth seals are commonly used to seal spool bearing compartments. Contact seals, generally used when relative spool surface velocities are low (less than 425 feet per second), possess favorable sealing characteristics, but have contact velocity limitations. Labyrinth seals, on the other hand, are generally used when relative spool surface velocities are high (greater than 425 f.p.s.). A disadvantage of labyrinth seals is that they do not seal as well as contact seals. In fact, the amount of buffer air that must be bled into the bearing compartment when labyrinth seals are used (versus contact seals) is appreciable.
Several disadvantages emanate from using buffer air as a sealing means. One disadvantage is that air bled off for sealing purposes is initially at a high pressure and temperature. Before the air can be used as buffer air, it must be cooled through a heat exchanger. A significant drop in pressure occurs when the buffer air passes through the heat exchanger, leaving a minimal difference in pressure across the bearing compartment air seals. If the pressure within the adjacent compartments drops below that in the bearing compartment, undesirable hot air can be drawn in from nearby compartments, or oil can be drawn out of the bearing compartment, or both.
Another disadvantage of using buffer air for sealing purposes is that air bled off of the compressor for sealing purposes cannot be used as efficiently as it might have been elsewhere in the engine. Work imparted to air by the compressor directly increases the work produced in the downstream turbine, and therefore the efficiency of the engine. Work imparted to air bled off for sealing purposes, in contrast, is substantially expended during the sealing process and therefore does not directly increase the work done within the turbine.
Another disadvantage of using buffer air for sealing purposes is that heat exchangers, oil/air separators, and their associated hardware add complexity, weight, and cost to the engine.
What is needed is an apparatus for sealing liquids between spools of a gas turbine engine that at does not use buffer air and one that does not require heat exchangers, oil/air separators, and hardware associated therewith.